Power loading transmit beamforming in MIMO-OFDM wireless communication systems

ABSTRACT

In a wireless communication system including a plurality of sub-carriers and a plurality of eigenbeams, a method for tuning a beamformed signal. The method includes adjusting a total gain of each of the plurality of sub-carriers and eigenbeams, and applying the adjusted total gain to each of the sub-carriers and each of the eigenbeams.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application No.60/782,459, filed Mar. 15, 2006 which is incorporated by reference as iffully set forth.

FIELD OF INVENTION

The present invention relates generally to the field of wirelesscommunications systems. More specifically, the present invention relatesto a method and apparatus for beamforming in a multiple-in/multiple-out(MIMO) orthogonal frequency division multiplexing (OFDM) wirelesscommunication system.

BACKGROUND

In multiple in-multiple out (MIMO) orthogonal frequency divisionmultiplexing (OFDM) wireless communication systems, transmit beamforming(TxBF) typically will improve signal-to-noise ratio (SNR) at a receiver.Transmit beamforming may provide a higher throughput and in turn allowfor higher data rates as compared to, for example, direct mapping orspatial spreading.

Channel state information (CSI) typically must be available at thetransmitter in order to employ TxBF techniques. A transmitter mayestimate CSI by assuming channel reciprocity, or the transmitter maydetermine CSI from a receiver by way of signaling. It should be notedthat channel reciprocity requires radio calibration which could beachieved by exchanging sounding packets. The transmitter may thenperform beamforming based on the estimation of received CSI and select aproper modulation and coding scheme (MCS) based on the MCS indexrecommended by the receiver through signaling.

Prior art wireless communication receivers utilize MCS indexes in orderto match code rate and modulation as close as possible to channelconditions. However, since the number of MCS indexes is limited, MCSindexes selected by the receiver may not closely match the existingchannel conditions. Prior art receivers select from a limited set of MCSindexes to match rate and modulations as closely as possible to channelconditions.

Therefore, it would be desirable to have a method and apparatus forredistributing power for all sub-carriers and eigenbeams within the setof selected MCS indexes. This would provide a fine adjustment to the MCSindexes in order to closely match current channel conditions.

SUMMARY

The present invention is a method and apparatus for beamforming inMIMO-OFDM wireless communication systems. In a preferred embodiment,power loading is used to redistribute power for all sub-carriers andeigenbeams, thus providing modulation and coding schemes (MCS) indexeswith fine adjustments in order to more closely match the current channelconditions. Performance is thereby increased in terms of decreasedpacket error rates (PER) and higher throughput.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding of the invention may be had from thefollowing description of a preferred embodiment, given by way of exampleand to be understood in conjunction with the accompanying drawing(s)wherein:

FIG. 1 shows an exemplary wireless system, including an access point(AP) and a plurality of wireless transmit/receive units (WTRUs),configured in accordance with the present invention;

FIG. 2 is a functional block diagram of an AP and a WTRU of the wirelesscommunication system of FIG. 11; and

FIG. 3 is a flow diagram of a method of power loading in accordance withone embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone (without the other features andelements of the preferred embodiments) or in various combinations withor without other features and elements of the present invention.

When referred to hereafter, the terminology “wireless transmit/receiveunit (WTRU)” includes but is not limited to a user equipment (UE), amobile station, a fixed or mobile subscriber unit, a pager, a cellulartelephone, a personal digital assistant (PDA), a computer, or any othertype of user device capable of operating in a wireless environment. Whenreferred to hereafter, the terminology “base station” includes but isnot limited to a Node-B, a site controller, an access point (Basestation), or any other type of interfacing device capable of operatingin a wireless environment.

Turning now to FIG. 1, there is shown an exemplary wirelesscommunication system 100 configured in accordance with the presentinvention. The wireless communication system 100 includes a plurality ofwireless communication devices, such as an AP 110 and a plurality ofWTRUs 120, capable of wirelessly communicating with one another.Although the wireless communication devices depicted in the wirelesscommunication system 100 are shown as APs and WTRUs, it should beunderstood that any combination of wireless devices may comprise thewireless communication system 100. That is, the wireless communicationsystem 100 may comprise any combination of APs, WTRUs, stations (STAs),and the like.

For example, the wireless communication system 100 may include an AP andclient device operating in an infrastructure mode, WTRUs operating inad-hoc mode, nodes acting as wireless bridges, or any combinationthereof. Additionally, in a preferred embodiment of the presentinvention, the wireless communication system 1100 is a wireless localarea network (WLAN). However, the wireless communication system 100 maybe any other type of wireless communication system.

FIG. 2 is a functional block diagram of an AP 210 and a WTRU 120 of thewireless communication system 100 of FIG. 1. As shown in FIG. 2, the AP110 and the WTRU 120 are in wireless communication with one another. Inaddition to the components that may be found in a typical AP, the AP 110includes a processor 215, a receiver 216, a transmitter 217, and anantenna 218. The processor 215 is configured to generate, transmit, andreceive data packets in accordance with the present invention. Thereceiver 216 and the transmitter 217 are in communication with theprocessor 215. The antenna 218 is in communication with both thereceiver 216 and the transmitter 217 to facilitate the transmission andreception of wireless data.

Similarly, in addition to the components that may be found in a typicalWTRU, the WTRU 120 includes a processor 225, a receiver 226, atransmitter 227, and an antenna 228. The processor 225 is configured togenerate, transmit, and receive data packets in accordance with thepresent invention. The receiver 236 and the transmitter 227 are incommunication with the processor 225. The antenna 228 is incommunication with both the receiver 226 and the transmitter 227 tofacilitate the transmission and reception of wireless data.

The present invention may be implemented in a WTRU or base station. Thepresent invention is applicable to both the physical layer (PHY) and thedigital baseband. The present invention may be implemented in wirelesscommunication systems employing the following air interfaces: widebandcode division multiple access (WCDMA), time division duplex (TDD),including HCR, LCR, and TDS-CDMA, frequency division duplex (FDD), andIEEE 802.11n air interfaces.

In a currently preferred embodiment of the invention, power loading inaccordance with the present invention is applied to the eigenbeamforming mode of a MIMO-OFDM wireless communication system.Preferably, power loading is only applied while closed loop powercontrol is in operation, and when accurate and recent CSI is availablefor use in preceding for eigen beamforming.

FIG. 3 is a flow diagram of a power loading method 300 in accordancewith one embodiment of the present invention. The method 300 begins byranking the eigenvalues of the channel correlation matrix using achannel estimation matrix per subcarrier as shown at step 302 andequation (1).(λ₁(k)>λ₂(k)> . . . >λ_(nT)(k));  Equation (1)At step 304, create eigenbeams (E₁, E₂, . . . , E_(nT)) by grouping theranked eigenvalues for all subcarriers according to equation (2).E _(i)={λ_(i)(1), λ_(i)(2), . . . , λ_(i)(K)} for i=1, 2, . . . ,nT  Equation (2)In equation (2), K is the number of sub-carriers, nT is the number ofeigenbeams/data streams, and λ_(i)(j) is the i^(th) eigenvalue of thej^(th) subcarrier.

In step 306, the average of the eigenvalues per eigenbeam is computedaccording to equation (3).

$\begin{matrix}{{\lambda_{i}^{av} = {{\frac{1}{K}{\sum\limits_{j = 1}^{K}{{\lambda_{i}(j)}\mspace{14mu}{for}\mspace{14mu} i}}} = 1}},2,\ldots\mspace{11mu},{{nT}.}} & {{Equation}\mspace{14mu}(3)}\end{matrix}$

In step 308, a gain a(i,j) is computed such that

${{a\left( {i,j} \right)} = {{\sqrt{\frac{\lambda_{i}^{av}}{\lambda_{i}(j)}}\mspace{14mu}{for}\mspace{14mu} i} = 1}},2,$. . . , nT and j=1, 2, . . . , K. At step 310, the gain a(i,j) iscompared to a threshold. If the gain a(i,j) is greater than a threshold,TH_(a), at step 312, a(i,j) is set equal to TH_(a). This puts a limit onthe gain and limits the power loading to very poor sub-carriers.

At step 314, a gain b(i) is computed such that

${{b(i)} = {G_{mod}G_{code}\sqrt{\frac{\lambda_{1}^{av}}{\lambda_{i}^{av}}}}},$where G_(mod) is a relative modulation order of i^(th) eigenbeam to thefirst/strongest eigenbeam and G_(code) is a relative channel coding gainof i^(th) eigenbeam to the first/strongest eigenbeam. By way of example,M-QAM modulation requires, approximately, an additional 5 dB by addingone more bit to a symbol. If the first/strongest eigenbeam uses 256-QAMand the second eigenbeam uses 64-QAM, G_(mod) is approximately 10^(5(N)¹ ^(64QAM) ^(−N) ² ^(256QAM) ^()/20)=1/√{square root over (10)}, whereN₁ ^(64QAM)=log₂ 64 and N₁ ^(256QAM)=log₂ 256. Likewise, G_(code) iscomputed based on code gain between two eigenbeams.

Since the total power with the new gains (a and b) must be the same asthe original power with unit gain, at step 316, the equation

$c = \frac{k{\sum\limits_{i = 1}^{nT}\lambda_{i}^{av}}}{\sum\limits_{i = 1}^{nT}{\sum\limits_{j = 1}^{K}{{b^{2}(i)}{a^{2}\left( {i,j} \right)}{\lambda_{i}(j)}}}}$is solved, resulting in a value for a variable c. At step 318, gaing(i,j) is computed for all sub-carriers and eigenbeams such that g(i,j)=√{square root over (c)}b(i)a(i, j) for i=1, 2, . . . , nT and j=1, 2,. . . , K. At step 320, the gain g(i,j) is applied to all sub-carriersand eigenbeams of long training fields (LTFs) and data OFDM symbols.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention. Themethods or flow charts provided in the present invention may beimplemented in a computer program, software, or firmware tangiblyembodied in a computer-readable storage medium for execution by ageneral purpose computer or a processor. Examples of computer-readablestorage mediums include a read only memory (ROM), a random access memory(RAM), a register, cache memory, semiconductor memory devices, magneticmedia such as internal hard disks and removable disks, magneto-opticalmedia, and optical media such as CD-ROM disks, and digital versatiledisks (DVDs).

Suitable processors include, by way of example, a general purposeprocessor, a special purpose processor, a conventional processor, adigital signal processor (DSP), a plurality of microprocessors, one ormore microprocessors in association with a DSP core, a controller, amicrocontroller, Application Specific Integrated Circuits (ASICs), FieldProgrammable Gate Arrays (FPGAs) circuits, any other type of integratedcircuit (IC), and/or a state machine.

A processor in association with software may be used to implement aradio frequency transceiver for use in a wireless transmit receive unit(WTRU), user equipment (UE), terminal, base station, radio networkcontroller (RNC), or any host computer. The WTRU may be used inconjunction with modules, implemented in hardware and/or software, suchas a camera, a video camera module, a videophone, a speakerphone, avibration device, a speaker, a microphone, a television transceiver, ahands free headset, a keyboard, a Bluetooth® module, a frequencymodulated (FM) radio unit, a liquid crystal display (LCD) display unit,an organic light-emitting diode (OLED) display unit, a digital musicplayer, a media player, a video game player module, an Internet browser,and/or any wireless local area network (WLAN) module

1. A method for tuning a beamformed signal inmultiple-input/multiple-output (MIMO) orthogonal frequency divisionmultiplexing (OFDM) wireless communications that use a plurality ofsub-carriers and a plurality of eigenbeams, the method comprising:creating eigenbeams by grouping the eigenvalues for all subcarriers;determining an average eigenvalue per eigenbeam; determining a firstgain according to the average eigenvalue relative to each individualeigenvalue; determining a second gain based on a relative modulationorder of each eigenbeam to a first eigenbeam and a relative channelcoding gain of each eigenbeam to the first eigenbeam; determining atotal gain of each of the plurality of sub-carriers and eigenbeams,based on the average eigenvalues, the first gain and the second gain;and applying the total gain to each of the sub-carriers and each of theeigenbeams.
 2. The method of claim 1, wherein determining a total gaincomprises determining a constant based on a sum of the average of theeigenvalues per eigenbeam relative to a sum of the first gain, thesecond gain and each eigenvalue.
 3. The method of claim 1 furthercomprising: comparing the first gain to a predetermined threshold; andsetting the first gain to the threshold on a condition that the firstgain is greater than the threshold.
 4. The method of claim 1, furthercomprising ranking the eigenvalues of a channel correlation matrix usinga channel estimation matrix for each sub-carrier.
 5. The method of claim1, wherein the wireless communications utilize closed loop powercontrol.
 6. A wireless transmit/receive unit (WTRU) configured to:create eigenbeams by grouping eigenvalues for all subcarriers of anorthogonal frequency division multiplexing (OFDM) wirelesscommunication; determine an average eigenvalue per eigenbeam; determinea first gain according to the average eigenvalue relative to eachindividual eigenvalue; determine a second gain based on a relativemodulation order of each eigenbeam to a first eigenbeam and a relativechannel coding gain of each eigenbeam to the first eigenbeam; determinea total gain of each of the plurality of sub-carriers and eigenbeams,based on the average eigenvalues, the first gain and the second gain;and apply the total gain to each of the sub-carriers and each of theeigenbeams.
 7. The WTRU of claim 6 wherein the WTRU is furtherconfigured to determine a constant based on a sum of the average of theeigenvalues per eigenbeam relative to a sum of the first gain, thesecond gain and each eigenvalue.
 8. The WTRU of claim 6, wherein theWTRU is further configured to: compare the first gain to a predeterminedthreshold; and set the first gain to the threshold on a condition thatthe first gain is greater than the threshold.
 9. The WTRU of claim 6wherein the WTRU is further configured to rank the eigenvalues of achannel correlation matrix using a channel estimation matrix for eachsub-carrier.
 10. The WTRU of claim 6, wherein the WTRU is furtherconfigured to utilize closed loop power control.